principles of mechanistic pavement design · main hypotheses of mechanistic pavement design •...

Principles of Mechanistic

Pavement Design

prof. Ezio Santagata

ANAS S.p.A. - Politecnico di Torino

Doha, December 11th, 2018

Presentation outline

• Historical perspective – AASHTO Design Method

• Main hypotheses of mechanistic pavement design

• Required data

• Practical implications

• Pavement design in Qatar

• Ongoing activities of the QSD-ANAS team

• Discussion

Historical perspective

• Originally developed in the 60s

• Based on the new concepts of:

• Structural Number (SN)

• Present Serviceability Index (PSI)

• Equivalent Single Axle Loads (ESAL)

• PSI depends mainly on:

• Smoothness (Slope Variance, SV)

• Rutting (Rut Depth, RD)

• PSI depends only marginally on cracking

• Main emphasis on load spreading capacity (stiffness)

AASHTO Design Method - Origin

Historical perspective

• Improvement of procedures

• Minor mechanistic components

• Introduction of reliability

AASHTO Design Method - 1993

• Limitations:

• Extension to extreme environmental conditions

• Extension to non-conventional and regional materials

• Extrapolation beyond actual traffic loadings

• Derived from the old style approach to pavement design:

subgrade protection (rather than true performance)

Need for a “physical” (mechanistic) approach to pavement design

Main hypotheses of mechanistic pavement design

• Vertical loads uniformly distributed on circular areas

• Layers of constant thickness on a semi-infinite half-space

• Full friction (adhesion) between layers

• Materials: homogeneous, isotropic and elastic

Multi-layer elastic model

Evaluation of structural response

(stresses, strains, displacements)

by means of adequate methods/algorithms

• Critical issues:

• Obtain input data (accounting for effects of external variables)

• Ensure consistency with hypotheses (materials and construction practices)

TESTING, MODELLING & QA/QC SYSTEMS

Layer 1

Layer 2

Layer 3

Interface 1

Interface 2

Interface n-1

Layer n

(homogeneous half-space)

h1, E1, ν1

h2, E2, ν2

h3, E3, ν3

hn = ∞, En, νn


• ASPHALT

Viscoelastic (rate- and temperature-dependent)

• Complex modulus (or ITT resilient modulus) formally considered as an elastic modulus

• Representative temperature and loading rate (frequency)

• Direct measurement or calculation by means of composition-dependent models

• Assumed values of Poisson’s ratio

• Dependency upon stress mode (e.g. uniaxial vs. bending vs. indirect tensile)

Assessment of materials properties (stiffness)

• UNBOUND MATERIALS

Non-linear elastic (stress- or strain-dependent)

• Resilient modulus formally considered as an elastic modulus

• Direct measurement for a wide combination of stress states and subsequent modelling

• Assumed values of Poisson’s ratio


Adhesion between layers

• Lack (or loss) of adhesion:

• Affects stress-strain distribution (and consequently fatigue life)

• Can lead to reduction of fatigue life and to debonding/delamination phenomena

• Can lead to surface cracking (top-down)

• Adhesion is strongly dependent upon tack coat and prime coat application

• Need for specific QA/QC and performance tests

Canestrari & Santagata (2005) Raab & Partl (2007)Roque et al. (2017)


Thickness uniformity and materials homogeneity

• Variability of thickness and homogeneity:

• Promotes the onset of relative permanent deformation (rutting and roughness)

• Amplifies dynamic loading which further contributes to relative permanent deformation

• Leads to creation of weak areas with corresponding local damage

• Need for tight control during production and laying

Santagata & Sciamanna (2015)Santagata & Barbati (2006)


Rationale of mechanistic (structural) design

• By means of transfer functions structural response can be related to:

• Limiting conditions = pass/fail approach

N/Nf (terminal damage)

single loading and conditions

• Performance = damage accumulation approach

D(N) = ΣNi/Nfi (progressive damage)

multiple loadings and conditions

• Classical transfer functions:

• FATIGUE CRACKING: tensile strain at bottom of the asphalt layers

• RUTTING: compressive strain on top of the subgrade

TESTING & FIELD OBSERVATION

Santagata et al. (2004)


Assessment of materials properties (transfer functions)

• ASPHALT – Fatigue life

• Generally expressed as a function of strain level (Wohler curve)

• Laboratory assessment to be adjusted to field performance (calibration)

• Direct measurement or calculation by means of composition-dependent models

• Dependent upon stress mode (e.g. uniaxial vs. bending vs. indirect tensile)

• Advanced modelling with Viscoelastic Continuum Damage (VECD) mechanics

Number of loadings to failure

Init

ial st

rain

(µ

m)

Miglietta et al. (2018)


Auxiliary information for mechanistic (structural) design

• Traffic loading - MONITORING & MODELLING

• Intensity, distribution and time growth

• Operative speed

• Models of various levels of complexity:

• Network-based models

• Relationship with social factors

• Environmental factors - MONITORING & MODELLING

• Temperature

• Moisture

• Models of various levels of complexity:

• TEMPERATURE: single temperature vs. continuous modelling

• MOISTURE: prevalent conditions vs. continuous modelling


Key points for the best use of mechanistic design methods

• The value of a mechanistic design method is mainly a function of its VALIDATION

• Quality of predictions depends on quality of input data

• “Cross-breeding” of different methods can jeopardize their predictive capacity

• Fundamental role of (laboratory) testing

• Necessary support of good construction practices and adequate QA/QC systems

Pavement design in Qatar

Reference documents

• Qatar Highway Design Manual 1997 (QHDM 1997)

• Qatar Strategic Transport Model (QSTM 2013)

• Interim Advice Note No. 016 (IAN 016, 2013) – Expressways → Stiffness ranges, no transfer function

• Interim Advice Note No. 019 (IAN 019, 2013)

• Qatar Construction Specifications 2014 (QCS 2014)

• Supplementary Guidelines for Pavement Design (2015) – Local Roads

• Qatar Highway Design Manual 2015 (QHDM 2015) → General “transition document” (refers to MEPDG)

• Interim Advice Note No. 101 (IAN 101, 2016)

More recent documents do not explicitly introduce a Qatar-specific procedure

Pavement design in Qatar

QHDM 1997

• Catalogue format but based on mechanistic principles

• Reference values of materials properties

• Explicit transfer functions (Austroads, adapted from Shell)

• Guidelines for traffic monitoring and modelling

• ESAL loading – suggested truck factor ranges (including overloading)

• Single reference temperature

• Lack of true validation

FATIGUE transfer function

RUTTING transfer function

Ongoing activities of the QSD-ANAS team

Assessment of relevant input data - MATERIALS

• ASPHALT STIFFNESS

• Calculation from binder rheology and volumetrics

• Effects of short-term and long-term ageing

• Calibration against direct measurements

• Neat and modified binders

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E-07 1.E-05 1.E-03 1.E-01 1.E+01 1.E+03 1.E+05

G*

[kP

a]

ω [rad/s]

Bitumen A - Original

Bitumen A - RTFOT

Bitumen A - PAV

Bitumen B - Original

Bitumen B - RTFOT

Bitumen B - PAV



• ASPHALT STIFFNESS AND FATIGUE

• Direct measurement and modelling

• Multiple configurations (uniaxial, 4-point bending, indirect tensile)

• Introconversion and adjustment for compaction level, rate of loading and temperature

• Mixes with neat and modified binders, including RAP and CR

1

10

100

1000

10000

100000

1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06

|E*|

MP

a

Reduced Frequency, fr=f*a(T)

E*(raw) - aT (fit)

E* (fit) - aT (fit)



• STIFFNESS OF UNBOUND MATERIALS

• Direct measurement and modelling

• Effect of moisture content and level of compaction

• Correlation with index properties

• Functional for the sub-layering of sub-base and subgrade

Santagata et al. (2001)

Sub

-bas

e

Mean stress (kPa) Deviatoric stress (kPa)

First stress invariant (kPa)

Sub

grad

e

Deviatoric stress (kPa)

Re

silie

nt

mo

du

lus

(kP

a)

Re

silie

nt

mo

du

lus

(kP

a)


Assessment of relevant input data - TRAFFIC

• Follow-up of data processing from classified traffic counts

• Reference: Qatar Strategic Traffic Model (QSTM)

• North Road (3 stations), Dukhan Road (1 station), Salwa Road (6 stations)


Assessment of available design methods - TRIALS AND IMPLEMENTATION

• In the absence of field data for calibration/validation:

• Back-analysis of QHDM 1997 and sensitivity evaluation

• Adaptation of Austroads 2017 (in continuity with QHDM 1997)

• Trial use of AASHTO MEPDG (U.S.A., 2008) and SAPEM (South Africa,2013)

• With field data: preliminary implementation

0.00

0.20

0.40

0.60

0.80

1.00

1.E+05 1.E+06 1.E+07 1.E+08 1.E+09

Term

inal

dam

age

, N/N

f

ESALs

Reference

Reduced E1

Reduced E1, Increased E2

Reduced E1, Increased E2, Reduced h1

T1 T2 T6T5T4T3 T6+


Challenges

• Characterization of non-conventional materials

• Recycled components (e.g. RAP and CR)

• Fine asphalt mixtures (ref. QCS 2018)

• Incorporate countermeasures for rutting and bleeding

• Include perpetual pavement option

• Retrieve and model reliable/ unbiased traffic data for all categories of roads

• Calibration and validation of transfer functions with field data

• Consistency with other reference documents adopted in Qatar

• Link mechanistic design to Life Cycle Cost Analysis (LCCA) and Pay Factors

• Dissemination and training

All stakeholders are invited to cooperate and share available data/information

Thank you

For further information please visit

the “Qatar Future Roads” website

http://www.q-roads.com.qa/

http://www.q-roads.com.qa/

principles of mechanistic pavement design · main hypotheses of mechanistic pavement design •...

Documents